A device for using an optical fiber as a probe for detecting disturbances and measuring a temperature or strain of the optical fiber is known. Such a measurement device uses a technology of monitoring the spectrum of Brillouin scattering light detected from the optical fiber, and can measure a deformation or a temperature change of an object to be measured by attaching the optical fiber to the object to be measured or disposing the optical fiber in the vicinity of the object to be measured.
For example, Non-patent Document 1 discloses a technology for measuring a temperature of an object to be measured by which a predetermined monochromatic light is inputted into an optical fiber, a Brillouin spectrum obtained from the optical fiber is detected, and the temperature is measured on the basis of the information concerning the peak frequency of the detected Brillouin spectrum (hereinafter referred to as “peak frequency”). The Brillouin scattering is a nonlinear phenomenon of light being scattered by interaction of light and acoustic wave in an optical fiber.
The measurement technology described in Non-patent Document 1 is based on a sensing principle according to which the Brillouin spectrum obtained by detecting the Brillouin scattering light changes depending on the temperature of optical fiber. In particular, the Non-patent Document 1 describes that the peak frequency of the Brillouin spectrum changes linearly with temperature in a temperature region close to 230 to 370 K.
On the other hand, Non-patent Document 2 describes that a graph representing the relationship between the peak frequency and temperature in a temperature region close to 60 K to 90 K has an extremal value and the line width (hereinafter referred to as “spectral line width”) of the Brillouin spectrum changes linearly with temperature.
In the field of strain measurements, for example, a strain gage using the dependence of electric resistance on strain is known for detecting strains generated as forerunners of abnormal states such as fracture or damage in a variety of structures. However, the problem associated with the measurement method using such a strain gage is that the measurements are easily affected by power loss or external electromagnetic interference. For this reason, strain measurements using the Brillouin scattering light in which an optical fiber or an optical fiber cable comprising an optical fiber was employed in a probe for disturbance detection have attracted attention in the field of such strain measurements. In particular, because strain measurements using the Brillouin scattering light are suitable for measuring strain distribution and enable measurements with a high resolution, they are expected to be used for deformation diagnostics of structures such as buildings and bridges. Such strain measurements using the Brillouin scattering light are based on a sensing principle according to which the peak frequency of Brillouin spectrum changes linearly with respect to the value of strain generated in the optical fiber cable by external forces.
For example, BOTDA (Brillouin Optical Time Domain Analysis) and BOCDA (Brillouin Optical Correlation Domain Analysis) are known as strain measurement methods using the Brillouin scattering light obtained from an optical fiber cable.
Patent Document 1 discloses as a strain detection probe an optical fiber cable in which an optical fiber and a wire material with a low thermal expansion are integrated by a coating material. This optical fiber cable is so configured that an optical fiber is integrated via the coating material with the wire having a low thermal expansion coefficient, thereby providing for high resistance to thermal expansion and thermal shrinkage and decreasing temperature disturbance. The strain measurements described in Patent Document 1 are assumed to be using the BODTA or the like. The BOTDA is a measurement method using backscattered light and has a distance resolution of about 1 m.
Further, Non-patent Document 3 describes a measurement method (BOCDA) that can realize a distance resolution of 10 cm or less with respect to the distance resolution of about 1 m attained with the BOTDA described in Patent Document 1. In both the BOTDA and the BOCDA, a two-end input system is necessary in which a probe light is inputted from one end of an optical fiber and a pump light is inputted from the other end of the optical fiber.
FIG. 1 shows a schematic configuration of a conventional strain measurement system of a BOCDA type using an optical fiber cable. The strain measurement system of a BOCDA type shown in the figure comprises a LD (laser diode) 101 serving as a light source, a coupler 102 that divides the light equally into two parts, an isolator 103 through which the light can pass in one direction but cannot pass in the opposite direction, an amplifier 104 that amplifies the light signals, a circulator 105 having three ports and serving for coupling to one port that is adjacent to another port, a PD (photodiode) 106, which is a light-receiving element, and an optical fiber cable 110 that includes only one optical fiber 111 serving as a light waveguide and functions as a sensor section. As described above, in the strain measurements based on the BOCDA method, a two-end input system is necessary in which a probe light is inputted from one end of the optical fiber 111 and a pump light is inputted from the other end of the optical fiber 111.
In the strain measurement system of the BOCDA type, the pump light and probe light are frequency modulated by cosine waves, and position resolution is performed by causing the induced Brillouin scattering only in a specific position (correlation peak). The Brillouin spectrum obtained by implementing frequency modulation of the generated pump light and probe light is a spectrum which comprises only the disturbance information in the position of the correlation peak where the pump light and probe light are correspondent in phase and the frequency difference between the two becomes constant. As a result, local strains can be measured.
Further, Non-patent Document 4 describes peak frequencies of Brillouin spectra in a variety of optical fiber cables and data concerning a temperature dependency, and a strain dependency (see FIG. 2).
Patent Document 1: Japanese Patent Application Laid-open No. 2001-12970
Non-patent Document 1: Marc Nikles, et al., “Brillouin gain spectrum characterization in Single-Mode optical fibers”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 15, NO. 10, October 1997
Non-Patent Document 2: L. Thevenaz, et al., “Brillouin gain spectrum characterization in optical fibers from 1 to 1000 K”, Technical Digest, 16th International Conference on Optical Fiber Sensors, Oct. 13-17 (2003), Tu2-2, p. 38-41
Non-Patent Document 3: Hodate Kazuo, Arai Hiroshi, “Enlargement of measurement range by a temporal gating scheme in BOCDA fiber optic distributed strain sensing system of time-division pump-probe generation”, TECHNICAL REPORT OF IEICE, OPE2004-224 (2005-02) (In Japanese)Non-patent Document 4: Kellie Brown, et al. “Characterization of optical fiber for optimization of Brillouin scattering based fiber optic sensor”, Optical Fiber Technology 11 (2005), p. 131-145